This invention relates to the field of friction materials and particularly to friction materials for brake linings, clutches, and the like.
Brakes for vehicles utilize a stationary member which is pressed against a rotating member. Friction between the stationary and rotating members converts the mechanical energy of the vehicle into heat. In order to provide good wear resistance and a suitable coefficient of friction between the two members, a lining is attached to the stationary member.
Brake linings are grouped into three classes based upon their materials of construction, namely high temperature organic material linings, sintered friction material linings (metallic linings), and semimetallic material linings. The high temperature organic material linings and the semi-metallic lining are widely used in automobile and truck brakes. These linings consist of filler materials bonded together in a matrix of natural or synthetic binders and resins such as polymer of cashew nut shell liquid, rubber, asphaltic base materials, and phenolic resin. The filler materials include reinforcement fibers such as asbestos, glass fibers, cotton, and steel wool. Heat absorbers such as iron and copper powder, and friction modifiers such as barytes, graphite, alumina, and mineral powders are generally included in the filler material to provide the coefficient of friction, wear resistance, and temperature capability required for specific applications.
The synthetic resins commonly employed for the matrix of brake linings are the phenolics. The phenolics have high temperature stability and yield a substantial quantity (ca. 50% of initial weight) of strong char. In addition, they are among the least expensive of the high temperature matrix materials. However, there is a continuing need to improve the high temperature capability of brake linings. This is particularly true for disc brakes which are replacing drum brakes in many automotive applications, and for truck brakes which encounter very high operating temperatures under certain driving conditions. Additionally, there is a continuing need to increase the life of brake linings in order to avoid the expense and down-time required to replace brake linings.